This invention relates in general to electrostatographic imaging systems, and in particular, to improved magnetically attractable developer materials, the process of making them, and their use.
Electrostatographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776, 2,277,013, 2,297,691, 2,357,809, 2,551,582, 2,285,814, 2,833,648, 3,220,324, 3,220,831, 3,220,833, and many others. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. The electrostatic latent image is then rendered visible by a development step in which the charged surface of the photoconductive element is brought into contact with a suitable developer mix.
One method for applying the developer mix is by the well-known magnetic brush process. Such a process can utilize apparatus of the type described, for example, in U.S. Pat. No. 2,874,063, and often comprises a non-magnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations of developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and lying substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating tube picks up developer mix from a supply source and returns part or all of this material to the supply. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer-mix pickup, brush formation, brush contact with the photoconductive element, brush collapse and finally mix release.
In magnetic-brush development of electrostatic images, the developer is commonly a triboelectric mixture of fine toner powder comprised of dyed or pigmented thermoplastic resin with coarser carrier particles of a soft magnetic material such as "ground chemical iron" (iron filings), reduced oxide particles, or the like. The conductivity of the ferromagnetic carrier particles which form the "bristles" of a magnetic brush gives some advantage over other modes of development. The conductivity of the ferromagnetic fibers or bristles provides the effect of a development electrode having a very close spacing to the surface of the electrophotographic element being developed. By virtue of this development electrode effect, it is to some extent possible to develop part of the tones in pictures and solid blacks as well as line copy. This ability to obtain solid area development with magnetic brush developing makes this mode of developing advantageous where it is desired to copy materials other than simple line copy.
However, most currently available ferromagnetic carrier particles have an electrical resistance which is too high to produce good quality solid arc development. The various commercial carrier particles generally lack adequate conductivity because of the presence of an insulating surface layer of iron oxide, grease or other contaminants. Efforts to remove such surface contaminants often result in particles which have an even higher electrical resistivity. For example, washing or solvent treatment of iron carrier particles in an effort to remove contaminants merely exposes the surface of the underlying iron to aerial oxidation. The new layer of oxide often has far greater resistivity than the original contaminants. Such an oxide coating can be removed; however, special after-treatment and precaution in storage and handling are required in order to avoid any further oxidation.
Electrostatographic carrier surfaces and carrier particles are generally made from or coated with materials having appropriate triboelectric properties as well as certain other physical characteristics. However, the carrier substrate as well as the surface thereof should not be comprised of materials which are so brittle as to cause either flaking of the surface or particle breakup under the forces exerted on the carrier during recycle. The flaking thereof causes undesirable effects in that the relatively small flaked particles will eventually be transferred to the copy surface thereby interfering with the deposited toner and causing imperfections in the copy image. Furthermore, flaking of the carrier surface will cause the resultant carrier to have non-uniform triboelectric properties when the carrier is composed of a material different from the surface coating thereon. This results in undesirable non-uniform pickup of toner by the carrier and non-uniform deposition of toner on the image. In addition, when the carrier particle size is reduced, the removal of the resultant small particles from the photoconductive plate becomes increasingly difficult. Thus, the types of materials useful for making carrier or for coating carrier, although having the appropriate triboelectric properties, are limited because other physical properties which they possess may cause the undesirable results discussed above.
Thus, there is a continuing need for a better developer material for developing electrostatic latent images.